The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems operating diffusion detection sequences and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other systems, such as higher field bore type systems, and is not limited to the aforementioned application.
Typically, in magnetic resonance imaging, a main magnetic field Bo is generated through an imaging region wherein is located a subject. Radio frequency (RF) coils transmit RF pulses into the imaging region exciting and manipulating dipoles within the subject. Gradient coils superimpose gradient magnetic fields on the main magnetic field in order to spatially and spectrally encode the excited dipoles.
Diffusion weighting sequences detect the movement of water in the subject on the cellular level. These sequences typically detect the movement of water on the order of a few microns, or about the distance it takes to cross a cell membrane. Echo planar imaging (EPI) sequences have been used to detect diffusion in a magnetic resonance scan. Although fast in data acquisition, EPI sequences tend to be very motion sensitive and have relatively low signal to noise ratios (SNR).
Rotating k-space diffusion sequences without phase encoding are also used. These sequences are typically more resilient to patient motion, and have higher SNR. However, they take much longer than an EPI sequence, typically on the order of three to four minutes. Another drawback of this type of sequence is that it tends to put heavy loads on the gradient equipment, and is not implementable in all present day MRI scanners. In addition, high, rapidly changing gradient fields can add to patient discomfort.
Fast spin echo (FSE) sequences have also been used to detect diffusion. Multi echo FSE sequences are faster than the rotating k-space sequences, but tend to have other limitations. Stimulated echoes and the primary echo of this type of sequence must have the same time and phase. In a normal FSE sequence, this is generally not a problem, but in a diffusion detection sequence, the phase becomes distorted unpredictably by patient motion and cannot be corrected. Thus, the primary echo and stimulated echoes cannot be summed properly.
Methods to restrict patient motion have been used, but are not desirable. Sedatives can be given to the patient to render the patient immobile, but there is always some degree of risk to the patient. Mechanical restraints are also used to restrict patient motion, but these are often imposing and uncomfortable to the patient. Such discomfort can often cause the patient to fidget and move more than if they were unrestrained.
The present invention provides a new and improved method a nd apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance is provided. Resonance is excited in selected dipoles of a subject in an imaging region. The resonance is refocused with an inversion pulse, and at least two gradient recalled echos are acquired. Data is collected until enough is present to be reconstructed into an image representation to the subject.
In accordance with another aspect of the present invention, a magnetic resonance apparatus is provided. A main magnetic assembly generates a main magnetic field through a subject in the imaging region. An RF coil assembly transmits RF pulses into the imaging region. A gradient coil assembly superimposes gradient fields on the main magnetic field inducing at least two gradient recalled echos. A receiver receives magnetic resonance signals from the imaging region, and a reconstruction processor reconstructs the received magnetic resonance into an image representation.
One advantage of the present invention resides in faster acquisition times.
Another advantage resides in higher signal to noise ratios in diffusion weighting imaging.
Another advantage resides in more robust data acquisition schemes.
An other advantage resides in reduced RF and gradient loads.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.